Plasma-enhanced chemical vapour deposition of silicon dioxide coatings is bringing new technology to future medical products and legacy devices
Silicon dioxide, or silica, is one of the most fundamental elements on earth.
Most commonly found in nature as quartz, it is also the major constituent of sand and a primary component in silicone and glass.
Now, this basic chemical compound is being applied using plasma-enhanced chemical vapour deposition (PECVD) techniques as an anti-microbial barrier, a primer to promote adhesion between stainless steel and proprietary coatings, or to create hydrophobic or hydrophilic surfaces.
For many medical device manufacturers, the application of proprietary coatings and surface treatments can play a major role not only in new product development, but also when upgrading legacy medical devices under 510(k) guidelines.
As a result, the medical device industry is aggressively investigating and applying plasma-applied coatings to products such as stainless steel guide wires, catheters, stents and vascular surgical tools.
“We are always looking for novel ways to make our products more robust but to do that we need to bring more technology to our devices. Often, that is going to involve some form of coating to functionalise the surface,” explains Aaron Baldwin of MicroVention, a company that offers a variety of neuro-interventional products including access products, intraluminal stents, occlusion balloons and polymer coils.
“PECVD can take a product to the next level by addressing surface reaction issues such as biocompatibility or lubricity. It is a unique and eloquent way to deposit and enhance coatings because it allows you to tailor the surface while retaining the bulk material’s properties you need,” adds Baldwin.
PECVD of organic silicones
PECVD is a process used to deposit thin films from a gas state (vapour) to a solid state on a substrate. To deposit silicon dioxide using PECVD, organic silicones are often required as the feedstock. Within this family, the best known are hexamethyldisiloxane (HMDSO) and tetramethyldisiloxane (TMDSO).
HMDSO, in particular, is an affordable and flexible reagent that is commercially available in a high purity, liquid form. The volatile, colourless liquid can be plasma-polymerised to create a variety of surface coatings that are safe for medical use. Depending on the composition of oxygen to HMDSO, the property of the surface can be hydrophobic or hydrophilic.
In fact, it is this flexibility that makes HMDSO and other siloxanes the ideal choice for PECVD. By adjusting the parameters and other gasses added, chemists can tightly control the material to address a wide range of applications.
For the medical device industry the primary uses of organic silicones fall into the primary categories of protective barriers (antimicrobial, antifungal, anti-corrosion), as a primer between stainless steel and exotic metals and proprietary surface coatings, or to modify the surface to become hydrophobic or hydrophilic.
When the substrate is metallic, such as stainless steel or other exotic alloys, it can be difficult to adhere a coating to the surface. When this is the case, HMDSO can be used as an intermediate layer to improve the adhesion between a coating and the substrate.
Guide wires, are a good example. To ease insertion, stainless steel guide wires are often coated with proprietary surface coatings to make them more lubricious. By applying a thin layer of silicon dioxide, the lubricious coating grafts very nicely to a stainless surface.
Organic silicones can also be applied as a linking chemistry between other difficult-to-adhere to surfaces such as ceramics and PTFE (Teflon). Drug delivery devices that utilise ceramic nozzles with micron-sized openings can become clogged and so are often coated with PTFE to prevent such an occurrence. Depositing a 100-150 nanometre layer of HMDSO promotes the bond between the two substances.
Anticorrosion is becoming increasingly important in medical devices, particularly to protect small microelectronic circuit boards used today in products or implanted in the body such as hearing aids, intraocular devices, implantable sensors and pacemakers.
To protect electronics against corrosion, HMDSO coatings are applied in a relatively thick coating of a micron or more. HDMSO is water and gas repellent – properties that are required to prevent corrosion. A thin layer (100 nanometres or so) of PTFE can also be applied if the HMDSO will be exposed to harsh chemical acids or bases.
For vascular surgical tools and instruments become fouled with tissue debris or blood, a plasma enhanced chemical deposition technique can provide a coating that keeps the surgeon’s tools cleaner, longer.
This is typically accomplished by applying a hydrophobic (water repellent) coating that repels water or biological fluids like blood.
When used on vascular surgical devices blood and tissue sheets off very easily so that the surgeon can see more easily when cutting, for example.
At the other end of the spectrum are hydrophilic (affinity to water) devices. Depending on what is required, organic silicones can be used to create such surfaces with either polar or dispersive surface energy.
Potential applications include coating polypropylene or polystyrene plates with alcohol or to facilitate protein bonding to the surface.
There are many strategies to achieve an anti-microbial surface including cell harpoons, amphipathic surfaces, antiseptics bound to the surface and non-stick coatings.
In a unique application, chemical vapour deposition is being used to embed nanosilver particles in a thin layer of organic silicone to prevent microbial adhesion and protect against corrosion.
Nanosilver, or colloidal silver, has been known for its antimicrobial effects from the earliest days of its use. Using a PECVD process, the tiny silver ions can be embedded in a thin layer of silicon dioxide to kill any bacteria present.
Despite the flexibility of PECVD-applied organic silicones, developing the precise chemistries, added gases and even plasma equipment design requires a close, collaborative relationship between medical device designers and equipment manufacturers.
Because MicroVention already had an established relationship with PVA TePla – several of its plasma chambers were already being utilised to aide in coating adhesion – Baldwin began consulting with the firm on a project to determine the benefits of coatings for stents.
According to Baldwin, plasma equipment manufacturers fall into two categories: those that produce commodity, off-the-shelf products and those that design and engineer systems to fit the needs of a specific application and/or to resolve unique surface energy challenges.
Companies such as PVA TePla are often tasked with the latter. In many ways, the application of plasma to meet unique surface requirements is the domain of chemists and other scientists. This is reflected in the accumulation of experts at the company, which includes three Ph.D scientists and surface, polymer, physical, bio and organic chemists, as well as engineers, plasma physicists and metallurgists.
When companies present PVA TePla with a challenging surface chemistry problem they are encouraged to visit the lab in California, USA. This gives them an opportunity to brainstorm with the technical team and run experiments together.
It is during these technical customer/supplier meetings that many of the best experimental matrices and ideas are produced. In addition to designing and manufacturing plasma systems, the company also serves as a contractor manufacturer and so has the in-house equipment to run parts and conduct experiments, with full customer involvement.
“When we start on something new, instead of poking around in the dark it is better to get expertise involved and [PVA TePla] is very willing to do experimentations – often free of charge – to get the project moving and improve the characteristics of the system and chemistries involved,” says Baldwin. “We were able to go there and work on the plasma machines to determine our parameters and evaluate the equipment.”
For companies such as PVA TePla every system is designed to meet the specific requirements of the application, which can include unique fixtures, possibly unique electrodes, and chamber modification to accommodate throughput and coating uniformity.
With organic silicones, the ability to thoroughly clean the chamber after each application is a major consideration as it coats the entire interior of the chamber (including the electrodes) in addition to the products receiving the coating. s a result, PVA TePla modifies the chamber to make it more easily cleanable by the end user after every coating application.